Formation of exotic baryon clusters in ultrarelativistic heavy-ion collisions

Recent experiments at the Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) have demonstrated that there are excellent opportunities to produce light baryonic clusters of exotic matter (strange and antimatter) in ultrarelativistic ion collisions. Within the hybrid-transport ultrarelativistic quantum molecular dynamics (UrQMD) model, we show that the coalescence mechanism can naturally explain the production of these clusters in the ALICE experiment at LHC. As a consequence of this mechanism, we predict the rapidity domains where the yields of such clusters are much larger than the observed one at midrapidity. This new phenomenon can lead to unique methods for producing exotic nuclei.

Figure 1

Transverse momentum spectra of protons, deuterons, and ${}^3\mathrm{He}$ particles in ${}^{208}\mathrm{Pb}$ on ${}^{208}\mathrm{Pb}$ collisions at the center-of-mass energy $\sqrt{s}=2.76$ TeV per nucleon. The symbols with systematical (thin boxes) and statistical errors are ALICE experimental data [31, 32] in the center-of-mass rapidity range from $-0.5$ to +0.5 and normalized per number of events for the 20% most central events. The UrQMD coupled with coalescence of baryons (CB) calculations of the same particle spectra at the same conditions are shown by short-dashed (blue), solid (red), and long-dashed (green) lines for the corresponding coalescence parameters $v_c$ (see in the figure).

Figure 2

Elliptic flows $\left(v_2\right)$ of produced protons and deuterons versus their transverse momenta. The reactions and the rapidity range are as in Fig. 1; however, the semicentral collisions (with the centrality range of 30–40%) are selected. The ALICE experimental data [33] for protons are the square symbols with errors within the thin boxes. The solid (green) and long-dashed (red) lines are the UrQMD and CB calculations for protons and deuterons, respectively. The short-dashed red line presents the scaled distribution for deuterons (see the text).

Figure 3

The total rapidity distributions of produced particles normalized per one event. (a) Normal baryons (black solid line), all composite nucleon fragments (red dashed line), and all composite hyperfragments (blue short-dashed line). (b) The same but for antiparticles. The UrQMD and CB calculations are performed for ${}^{208}\mathrm{Pb}$ on ${}^{208}\mathrm{Pb}$ collisions at $\sqrt{s}=2.76$ $A$ TeV overall impact parameters (minimal bias).

Figure 4

The same as in Fig. 3, however, only for normal particles. (a) Light clusters with sum baryon numbers $A=2$, 3, and 4 (black solid, red dashed, and blue short-dashed lines, respectively). (b) Rapidity distributions of individual particles: deuterons, tritons, and hypertritons given by black solid, red dashed, and blue short-dashed lines, respectively.